(1) Field of the Invention
This invention relates to the suspension of propulsion engines to an aircraft structure. It relates in particular to a suspension or rear fastening device including means for retaining the suspension in case of accidental breakage of one of its parts.
(2) Description of the Related Art
A propulsion engine can be mounted at various places of an aircraft, for example suspended from a pylon integral with the structure thereof. It may be suspended under the wings, fastened to the fuselage or mounted in the tail unit by attachment means. The purpose of these attachment means is to ensure transmission of the mechanical loads between the engine and the aircraft structure. The loads to be taken into account are oriented along the three main directions. These are notably the weight of the engine, its thrust and the lateral aerodynamic loads. The loads to be transmitted also include the recovery of the rotation torque around the axis of the engine. Besides, these means must absorb the deformations undergone by the engine during the various phases of the flight by reason notably of the dimensional variations due to thermal expansions or contractions.
A suspension mode, for example, in the case of a fan-type turbo engine, consists in fastening the engine to a pylon integral with the wing structure of the aircraft using a front suspension or attachment and rear suspension or attachment. The front suspension is fastened in particular to the outer casing of the fan and the rear suspension to the exhaust casing of the primary flux.
According to a configuration known, the front attachment is arranged to ensure transmission of the vertical and tangential loads between the engine and the aircraft. The rear attachment is arranged for letting through the mechanical loads along the same directions, as well as to enable recovery of the engine torque around the axis of the turbo engine and the thrust recovery. The latter is transmitted via two thrust recovery bars attached, at the front, to the base of the fan casing on both sides of the longitudinal axis and, at the rear, by means of the rear suspension of the engine.
The present invention relates to an attachment arranged for transmitting the vertical and lateral loads between the engine and the pylon of the aircraft. It is the configuration of rear attachment reminded above.
The patent EP 527672 of the applicant describes an attachment of such type. It includes three outer attachment lugs or yokes which are provided as a single-piece at the upper section of the exhaust casing of the turbo reactor. These lugs enable to fasten the exhaust casing to the lower ends of three suspension rocker bars whereof the upper ends are mounted in an attachment structure arranged substantially as an arc of circle on a transversal plane with respect to the axis of the engine. This attachment structure includes a brace to which the rocker bars are attached via a longitudinal suspension axle. This brace is itself fixed under the pylon by appropriate means. Thanks to their arrangement, the rocker bars enable transmission of the traction and compression loads along their axis. The rocker bars are fixed at their other end to lugs or yokes of a lower brace integral with the exhaust casing.
The solution suggested in this patent relates to a means enabling to prevent the engine from coming apart in case when said brace should break mechanically.
The attachment devices also integrate generally safety means intended to prevent the engine from coming apart when a rocker bar, and not the brace, breaks. For example, we know means composed notably of elements which remain inactive under normal operating conditions of the engine, i.e. when the parts of the attachment device are intact. If either part breaks, the safety means become active. The elements take over from the faulty parts in the attachment device.
We know for example an attachment device, such as described in the patent U.S. Pat. No. 6,330,995, including a first connecting rod and a second connecting rod between both upper and lower braces. The first connecting rod is mounted using a ball joint connection to a lug of the engine, and using two distinct links to the upper brace whereof one is fitted with a ball joint, the other one is axial. The second connecting pin is associated with a lug of the engine via a ball joint connection and to the upper brace via a single ball joint connection under normal operation. A second connection is provided, but for the latter the connecting pin is mounted to its counterpart with a backlash. Under normal operation, this second connection remains therefore inactive. The device includes a third connecting rod, between both braces, which remains in stand-by also by the presence of backlashes between the connecting pin and their respective housing on the braces. Under normal operation, the transmission of the loads is performed by the first of the connecting rods by reason of both its connecting pins with the upper brace. The second connecting rod transmits the tension and contraction loads by reason of its assembly with a single connection.
If the second connecting rod breaks, both braces pivot with respect to one another around the first connecting rod in order to eliminate the initial backlashes on the third connecting rod. The latter becomes actives and replaces the second connecting rod. If the first connecting rod breaks, the backlashes of the second connecting rod are eliminated as well as the backlash of the third link of the second connecting rod. There again, the suspension fulfills its function further without any alterations in its performances.
It may be observed that the first and second connecting rods are in the form of a boomerang and exhibit certain symmetry. They are, however, not interchangeable since one of the axes of the second connecting rod is mounted with a backlash whereas the three axes are active in the first.
This type of arrangement involves machining specific parts. Each connecting rod has a unique position in the suspension. There are therefore as many references as parts forming the attachment device. This does not imply optimum savings in manufacture and management.
Besides, both main connecting rods are quasi identical in shape. During assembly, the technician must adhere to very accurate operating mode and proceed with greatest care. The risk of mistakably assembling a part instead of another is not negligible.
The applicant has set himself as a target to provide a device for attaching an engine to the aircraft structure, whereof the risks of error during assembly are brought back to a minimum.
It also aims at the realisation of a device including interchangeable parts. It would be therefore possible to perform the assembly with a number of different parts smaller than in the solutions of the previous art. Not only would the risk of error be reduced, but also the costs of manufacture, maintenance and stock management.